Since the power of an electromagnetic wave generally attenuates in accordance with the propagation distance, the communication quality deteriorates in accordance with the distance from a transmitting apparatus in wireless communication. Also, if a transmitting apparatus or receiving apparatus is hidden behind an obstacle, the electrical power decreases even when the communication distance is small, and the communication quality deteriorates in the same manner as when the communication distance is large.
As a method of preventing the deterioration of the communication quality as described above, a relay apparatus can be deployed between a transmitting apparatus and a receiving apparatus. By relaying wireless signals by deploying the relay apparatus, it is possible to shorten the propagation distance between wireless communication apparatuses, and decrease the probability that the communication quality degrades due to shadowing wherein a communication apparatus is hidden behind an obstacle.
Conventional wireless communication methods can roughly be classified into two types: (1) amplify-and-forward (referred to as the AF method hereinafter); and (2) decode-and-forward (referred to as the DF method hereinafter).
In the AF method, the relay apparatus does not restore transmission data from a received signal, but directly amplifies the received signal and relays the amplified signal to a receiving apparatus.
In the DF method, the relay apparatus restores transmission data from a received signal, and relays the data. Since the transmission data is encoded for error detection, the relay apparatus interrupts relay if it detects an error (occurring in a link between a transmitting apparatus and the relay apparatus). The purpose of this processing is to prevent the relay apparatus from relaying an error signal.
Comparing the AF method with the DF method shows that the DF method can theoretically achieve a higher throughput.
Note that in either method, a diversity gain can be obtained if not only signals can be received via the relay apparatus but also signals transmitted by a transmitting apparatus can directly be received by a receiving apparatus. Note also that in the DF method, a coding gain can reportedly be obtained by applying different coding schemes on the transmitting apparatus and the relay apparatus (e.g., T. E. Hunter and A. Nosratinia, “Cooperation Diversity through coding,” IEEE ISIT 2002, Lausanne, Switzerland, p. 220, 2002.).
As described above, if an error occurs in the reception of a signal transmitted by a transmitting apparatus in the conventional relay apparatus using the DF method having a high throughput, the relay apparatus does not relay the erroneous received signal but discards it. In the conventional wireless relay system, therefore, the communication efficiency decreases if the relay apparatus does not correctly receive a signal transmitted by a transmitting apparatus and an error occurs.
Also, if the coding rate is set low, a low-order modulation method is allocated, or the transmission power is increased in order to prevent an error in the relay apparatus, the frequency use efficiency decreases, and the communication efficiency decreases.
In addition, demands have arisen for low power consumption and size reduction of the relay apparatus and a receiving apparatus.
As described above, the communication efficiency conventionally decreases if a signal received by the relay apparatus contains an error.